1. Field of the Invention
The present relates to the manufacture of semiconductor devices. More particularly, the present invention relates to techniques for improving the resolution of photoresist development in deep ultra violet photolithography processes.
2. Description of the Related Art
In the manufacture of semiconductor integrated circuits, many well known photolithography techniques are used to pattern the various functional features on different levels of an integrated circuit chip. Generally, photolithography involves selectively exposing regions of a photoresist coated silicon wafer to a light radiation pattern, and then developing the exposed photoresist in order to selectively protect regions of wafer layers, such as metallization layers, oxide dielectric layers, polysilicon layers, silicon layers, etc., from subsequent etching operations.
As is well known, photoresist is a light radiation-sensitive material that is typically spin-coated over a selected layer of a silicon wafer. The photoresist material is classified as either positive or negative depending on how it chemically reacts to light radiation during exposure. Positive photoresist, when exposed to radiation becomes more soluble and is thus more easily removed during the development process. In contrast, negative photoresist will generally become less soluble when exposed to radiation, thereby enabling the removal of non-exposed regions. Although traditional I-Line photolithography process works well for patterning features in the 0.35 micron technology and larger, as feature sizes in integrated circuits continue to shrink, the patterned photoresist has been exhibiting a number of resolution abnormalities.
To address this limitation, engineers have been implementing optics that enable the use of wavelengths that are shorter than traditional "I-line" wavelengths (i.e., 365 nm). For example, a number of common photolithography processes are now implementing deep ultra violet "DUV" wavelengths (e.g., 248 nm). As a result of implementing DUV wavelengths in the photolithography process, a number of chemically amplified DUV photoresists were also developed to better interact with the shorter wavelengths. Although the new chemically amplified DUV photoresists work relatively well, as smaller and smaller feature sizes are designed for higher performance integrated circuit devices, the resolution of these high performance designs have been exemplifying less than acceptable resolution due to poor photoresist development.
To exemplify this problem, FIG. 1A shows a photoresist patterning system 100, which illustrates a simplified DUV photolithography process. In this example, a semiconductor substrate 102 having several layers fabricated thereon is shown having a DUV photoresist layer 112. The semiconductor substrate 102 has an oxide layer 104, a titanium nitride layer 106, an aluminum metallization layer 108, and a titanium nitride layer 110. The layers 106, 108 and 110 are commonly referred to as a "metal-stack," which is patterned to define the electrical interconnection layout of a metallization layer. After one metal-stack is patterned, other subsequent metal-stacks may be patterned and interconnected using conductive contacts, thus forming a network of interconnect structures.
In order to pattern the metal-stack, the photoresist layer 112 is selectively exposed to the DUV light 120. To accomplish the selective exposure, the DUV light 120 is passed through a reticle 114 which has non-transparent regions 116. When the exposed photoresist is a "positive" DUV photoresist, exposed regions 112a will undergo a chemical reaction that generates an acid component. It is this acid component that catalyzes further acid generation, making exposed regions 112a more soluble than the non-exposed regions 112b during subsequent development. Typical development steps generally include performing a post exposure bake (PEB) on a hot plate, and then applying a developer solution.
FIG. 1B shows the metal-stack of FIG. 1A after the photoresist layer 112 has been developed. Ideally, all of the photoresist layer 112 lying in the exposed regions 112a should be vertically developed down to the titanium nitride layer 110 interface. Unfortunately, a residue of photoresist remains near the lower portions of the unexposed regions 112b producing what is known as a "footing" effect and other profile abnormalities. Specifically, footings 124 remain near the interface of the titanium nitride layer 110. In addition, a photoresist film 126 also sometimes remains on the surface of the titanium nitride layer 110 between the regions 112b. In the case of DUV photoresists, the footings 124 and film 126 are believed to result due to a loss of acid in the regions 112a after exposure and before development.
One reason for the acid loss in the exposed regions 112a is believed to occur when free surface nitrogen species of the titanium nitride layer 110 behave as a "Lewis base," which are partially neutralized by the generated acid component of the exposed photoresist layer 112. These nitrogen species are believed to contribute to the undesirable acid loss, which causes additional profile abnormalities that reduce the resolution of the developed photoresist 112.
Of course, when the photoresist layer 112 fails to develop as ideally desired, the etching of the underlying metal-stack will not reflect the desired patterns. As shown, if the photoresist had been ideally developed, lines 132 would most likely define the etched profile of the metal-stack. However, when the footings 124 result, lines 130 will define the true etched profile, which may fail to produce desired circuit performance specifications. The footing also degrades the profile control making the metal stack edge more sloped.
FIG. 1C shows a more detailed view of the interface between the titanium nitride layer 110 and the aluminum metallization layer 108 of FIGS. 1A and 1B. Typically, after the titanium nitride layer 110 is sputtered in the PVD chamber, the wafer is moved to a photoresist applicator station, where a liquid photoresist material is applied over the titanium nitride layer 110 and then spin coated. However, during the time in which the wafer is moved from the PVD chamber to the photoresist applicator station and the liquid photoresist is applied, general ambient oxygen "O.sub.2 " will come into contact with the titanium nitride layer 110. Because the low pressure PVD chambers sputter the titanium nitride layer 110 forming a sparse molecular structure, some of the oxygen atoms are known to migrate through the titanium nitride layer 110 and come into direct contact with the aluminum metallization layer 108. The chemical interaction between the oxygen atoms and the aluminum will generally produce a film of aluminum oxide (Al.sub.2 O.sub.3) 109. A drawback of the produced aluminum oxide film 109 is that a higher level of resistance is introduced into subsequently formed interconnect structures.
FIG. 1D illustrates a simplified interconnect structure between a first metal-stack 113 and a second metal-stack 123. As is well known, the various metallization layers are interconnected through conductive contacts 115 to enable conduction between, for example, point A and point B. Unfortunately, the aluminum oxide film 109 will also introduce high resistance in the second metal-stack 123, thereby further increasing the resistance between points A and B. Consequently, circuit designers are forced to accept and account for this performance limiting drawback when new circuit designs are made.
In view of the foregoing, there is a need for methods and apparatus for improving the retention of generated acid during DUV photolithography processes in order to reduce resolution abnormalities in developed photoresist layers. There is also a need for techniques for reducing the formation of high resistance films in interconnect metal stacks during fabrication processes.